In anticipation of the Tokai-Tonankai Earthquake and the Nankai Great Earthquake that are thought to occur in the future, borehole-type strain meters are presently buried and installed in many observation points in Japan (30 locations or more). The strain meters are used by multiple research institutions and government agencies including the present applicant for earthquake prediction research.
The borehole-type strain meter is placed inside a borehole (boring hole) bored in the ground surface and is fixed with a grout, whereby the strain meter is buried and installed in underground bedrock. Thus, the strain meter continuously senses fluctuation of strain of the bedrock (expansion and contraction of bedrock) over a long period of time.
Each of Patent Document 1 and Non-patent Document 1 describes a strain amplification sensor that has a strain amplification mechanism for continuously amplifying the strain using the principle of leverage. The strain amplification sensor senses a displacement amount of a tip end of a lever, which is amplified by the strain amplification mechanism, with a displacement sensor. The strain amplification sensor is incorporated in and integrated with a cylindrical case and used as a borehole-type strain meter.
Patent Document 2 describes a borehole-type strain meter that has a cylindrical case filled with a fluid (silicone oil) and that senses deformation of the case based on a displacement amount of the fluid.